Phonograph pickup with mechanical filter



June 21, 1949. KALMUS 2,473,650

I PHONOGRAPH PICKUP WITH MECHANICAL FILTER Filed June 10, 1943 2 Sheets-Sheet 1 lIA gc P ER POWDERED IRON IIBV FIG. 2

'5 WORKING 5' /AX|$ FW z F0 INVENTOR Y I HENRY P. KALMUS |.a 0 BYW )2- %1m HIS ATT ORNEY MOTION (MM) June 21, 1949. H. P. KALMUS 2,473,650

PHONOGRAPH PICKUP WITH MECHANICAL FILTER Filed June 10,1943 2 Sheets-Sheet 2 /C0PPEE Patented June 21, 1949 UNITED STAT PHONOGRAPH PICKUP WITH MECHANICAL FILTER Henry P. Kalmus, Chicago, 111., assignor to Zenith 2 Claims.

This invention relates to phonograph pickups.

In many instances it is desirable to move an element of an electrical circuit and to produce linear electrical variations in the circuit in ac cordance with linear movement of the element. This is particularly true in frequency modulation apparatus wherein the element may form one element of a tuned circuit, and especially where it is desired that linear movement of the element a p r nh no raphic, arm which ac s as a filter for reproducing undulations having frequencies above the frequencies corresponding to wow, and rejectin thosefrequencies which correspond to wow in phonograph records.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, both as to its organization and manner of operation, together should cause linear changes in resonant frequency with further objects and advantages thereof, may of a circuit over a range corresponding to the best be understood by reference to the following range traversed by the instantaneous intensity description taken in connection with the accomof 'a signal in accordance with which there is dispany r w n s in which: placement of such element. Figure 1 shO apparatu which may be used In a phonograph pickup unit it is desirable in l in Co j nct n W h invention; many instances to reproduce lateral, or hill and 2 S WZ appa atus for receiving signals sent dale, recordings by varying the operatin frein r nce th my invention; quency of an associated electrical circuit. In that 3 is a aph cal r presentat on of certain case the instantaneous resonant frequency of the h er ics of my nvention; circuit should bear a linear relationship to the 4 s a pr ctical m o m nt of one form amount of lateraLor hill and dale, displacement of my invention; of a stylus in a groove of the recording. 5 is a de view in elevation of a portion Still another problem is that laterally cut re- 0f t e apparatus Sh in Fi 4; cordings may be reproduced when they are 6 ShOWS S l another embodiment of my mounted eccentrically on the rotating spindle and invention; hill and dale recordings may be reproduced when 7 hOWS a modified form of the invention, the recording is in a warped condition. In such as i l r ed in F 6; cases where the reproducing, or tone, arm trans- Figs- 3 a 9 s w S ll another embodiment of lates the record groove undulations into a frey v n, F being asectional view on line quency change, very large changes of frequency 39 88 of Fig. 9 nd 9 bein a Sectional V ew may be produced at a low speed or at a low fretaken on l e 9 of Fig. 8; quency less than 100 cycles, by such eccentricity Figs- 10 and 11 Sh w y t another embodiment or warping. Such large, low frequency, frequency of my inventifln, being a sect onal View deviations are undesirable primarily because of taken Substantially O line I f Fig- 11 and limitations in most frequency modulation repro- Fig. 1 bei g a Sectional View taken Substantially ducers, which commonly include a frequency on line of 10. deviation responsive device or frequency discrim- Referring to the drawings wherein like referlnator which produces a voltage whose instan- EH66 numerals designate like parts in all fi u s, taneous intensity is linearly proportional to the F .1 sho s aradi fr q y tra s tt a ap d instantaneous frequency deviation of a wave, but to transmit f eq en y mo ul ted Si nals to a freonly over a limited frequency range. It is therequency modulation receiver, such as that shown fore desirable that the effect of such eccentricity in Fig. 2. The frequency of the wave transmitted or Warring up t qu y d vi i n pr from antenna In is determined by the reactances duced by the tone arm should be eliminated. of the oscillator resonant circuit including in- An ob ect of my invention is to provide a new ductance II in parallel with a circuit in which a d proved p ono aph p ck p. parallel connected capacitances 12 and is are in Another object of my invention is to provide a series with capacitance M. The discharge device new and p e P p u t os esponse I5 is connected in a modified Colpitts oscillator bears a linear relationship to record undulations i t, having frequencies within the audio range but It is apparent that, if the inductance of l which is practically nonresponsive to frequencies H is varied, the frequency of oscillation is also corresponding to wow, that is, eccentricity or varied. Variable capacitance l2 servesas a trimwarping, in phonograph records. ming capacitance for setting the Oscillator at a Another object of this invention is to provide predetermined frequency, about which it varies as the inductance II is caused to vary. Coupling capacitance l6 serves to maintain one terminal of the previously described resonant circuit at the high frequency potential of the anode of discharge device l5, While blocking the flow of continuous current from the anode to the grid, or control electrode, of the discharge device I5. The other terminal of the resonant circuit is connected through condenser l9 to the grid of device 5 and the cathode is connected between capacitances l3 and M. Energy is supplied to the oscillator from voltage source or battery I! which is connected in a series circuit with choke coil l8 between the anode and cathode of discharge device I5. The antenna I is connected directly to the anode of discharge device l and radiates energy having a frequency determined by the resonant circuit heretofore described.

In the frequency determining circuit the coil I is made with small inductance and the capacid tances l2, l3 and M are of relatively high capacity, the two reactances resonating at the frequency of the wave transmitted. Capacitances l2, I3 and I4 are purposely made large so that the effects of varying amounts of stray capacitance are relatively small, as when the coil H is touched or moved manually. Resistance 29 connected between the grid and cathode of discharge device I5 serves to maintain the grid of tube l5 at a definite direct potential with respect to its cathode and thus stabilizes the operation of the discharge device. Coil II is adapted to move, as indicated by the arrows in Fig. 1, with respect to the copper and powdered iron discs HA and |B in a manner hereinafter described.

The apparatus shown in Fig. 2 serves as a receiver and frequency discriminator for frequency modulation signals, which are received from antenna |9 in Fig. 1. Continuous currents proportional to frequency changes in the mean frequency of the carrier wave transmitted from antenna H] are indicated by direct current meter 2| and alternating voltages of frequency corresponding to the frequency of changes in the carrier frequency appear across the terminals 22 and 23.

The particular receiver or discriminator circuit shown in Fig. 2 is similar to one of a wellknown type, described on page 197 of Frequency Modulation by A. Hund, McGraw-Hill Book Company, Inc., 1942. In brief, frequency modulation signals are received by antenna 24 and are impressed across a radio frequency transformer having a primary winding 25 and a secondary winding 26 tuned respectively by capacitances 2! and 28 to the mean or carrier frequency about which there are excursions of instantaneous carrier frequency. Coils 25 and 26 are loosely coupled and are excited at the frequency at which the oscillator of Fig. 1 operates when coil II is in its normal, or undisplaced, position. At that frequency, the voltage appearing across coil 25 is 90 out of phase with the voltage appearing across coil 26.

The purpose of the receiver or discriminator is to produce signals corresponding to frequency variations imposed upon the carrier frequency in response to motion of coil II which changes the carrier from its means frequency. In order to cancel out effects due to amplitude variations of the carrier, the receiver or discriminator is made differentially acting by supplying a voltage, substantially in phase with the voltage appearing across coil 25, to the center tap of secondary winding 26. This in practice is done by connecting capacitance 33 of low reactance between antenna 24 and the center tap of winding 26. With this connection, and with the cathode of diode 29 grounded for high frequency currents through capacity 35, a voltage appears across diode 29 which is equal to the vectorial sum of the voltage between antenna 24 and ground and half the voltage induced in coil 26. The opposite ends of coil sections 26A and 26B are connected respectively to the anodes of diodes, or unidirectionally ucting devices, 29 and 30. The cathodes of rectifier devices 29 and 30 are grounded for high frequency currents, due to the fact that capacitance 35, of low reactance at the carrier or mean frequency, is connected between the cathodes and to ground. Also, the cathodes of rectifier or discharge devices 29 and 30 are connected in a series circuit including the equal resistances 3| and 32, serially connected. Lead line 36 connected between the center tap of coil 26 and the connection between resistances 3| and 32 represents a low resistance path for direct current flow between the output elements of the discharge devices 29 and 39.

With the balanced circuit thus far described it is clear that, when a signal is applied to antenna 24 with a frequency corresponding to the unmodulated carrier or mean frequency, the unidirectional rectified potentials appearing across equal resistances 3| and 32 are equal and are of such polarity that their net effect is zero on a continuous current meter 2|, which is connected in parallel with the series circuit including those equal resistances. Resistance 34 in series with meter 2| serves to decrease the sensitivity of meter 2 I.

When a carrier wave of shifted frequency is received on antenna 24, rectified direct potentials appearing across resistances 3| and 32 are of unequal intensity and their net effect is to cause current to flow through meter 2| in a degree and direction depending upon the degree and amount of frequency shift of the carrier wave.

The inductance of coil in Fig. 1 depends upon its position with respect to the discs of copper and powdered iron. Correspondingly, the frequency of radiation transmitted by antenna I0 is dependent upon the position of coil The receiver and discriminator shown in Fig. 2 is so balanced that no current flows in meter 2| when coil II is equidistant from the copper disc and the powdered iron disc shown in Fig. 1. Displacements of coil II from the equidistant position cause corresponding deflections on instrument 2|.

The inductance coil changes in inductance in a predetermined manner with its displacement. This is accomplished by mounting the 'coil so that its magnetic field encompasses a member having magnetic permeability greater than air and also encompasses a member having low resistance for the flow of electric current. In the absence of the powdered iron disc shown in Fig. 1, as coil moves toward the copper disc, the effective inductance of coil decreases. In the absence of the copper disc shown in'Fig. 1, as the coil moves toward the powdered iron disc, the effective inductance of coil increases. Changes in effective inductance of coil cause corresponding changes in frequencies transmitted by antenna l0.

This behavior is best illustrated in Fig. 3 wherein curve A represents the variation of transmitted frequency, in the direction of the arrow, when coil II is moved away from the o p disc, he po dered ir n isc being abse t. ur e B represen s the variation of transmitt d requency, n the direction o the arrow, as the coil II is moved away from the powdered iron disc, the copper disc being absent. Ordinate F0 represents the frequency of transmission when both the copper disc and the powdered iron disc are absent. Curve C represents the variation of transmitted frequency when both copper and powdered iron discs are present and as coil II is moved relative to both the copper disc and powdered iron disc. Ordinate FW represents the carrier, or mean, frequency at which the transmitter in Fig. I normally operates when coil H is in its normal or undisplaced position, and the meter 2| in Fig. 2 reads zero.

When interpreting Fig. 3 it is helpful to bear in mind that, when coil H is moved toward one of the discs (either copper or powdered iron), it moves away from the other disc. Curves A and B in Fig. 3 are nonalinear and curve A has a positive curvature, whereas curve B has a negative curvature about equal to the positive curvature of curve A. That is, when only one of the discs is employed the antenna [0 radiates frequencies not linearly proportional to displacement of coil II, and that non-linear proportionality is opposite for the two members HA and 11B. However, when both the copper disc HA and the powdered iron disc IIB are used, so

that the inductance of coil ll changes upon movement in accordance with curve C, linear movements of coil ll cause corresponding substantially linear changes in frequency of the wave radiated by antenna Ill.

Of course, every physical inductance coil possesses the properties of inductance and resistance. The magnitude of both of these properties of the coil are aifected by the presence in the alternating magnetic field thereof of bodies p e sin e ther magnetic or electricity conducting properties. That is, when a body such as copper is disposed in the alternating magnetic field of the inductance coil, due to thefiow of eddy currents, the inductance of the coil is reduced but the effective resistance of the coil is increased and, consequently when and as the coil moves away from the copper body the coil inductance is increased but the effective coil resistance is decreased. Also, when a body such as a powdered iron core is disposed in the alternating magnetic field of the inductance coil, due to the permeability and hysteresis loss in the iron core the inductance of the coil as well as its resistance is increased and consequently, when and as the coil moves away from the iron core the coil inductance as well as the coil re-. s stance is decreased. With these fundamental principles in mind, it is noted that in each one of the embodiments of my invention shown herein, where there is relative movement between an inductance coil and means (magnetic and conductive bodies) possessing magnetic and electricity conducting properties respectively, the inductance is increased or decreased as the case may be, depending upon the direction of relative movement, in a cumulative manner, but the efiective resistance, being changed in a dilferential manner, tends to remain substantially constant during such relative movement. Inasmuch as the effective resistance of the relatively movable coil tends to remain constant, the modulation of the oscillator circuit including said coil is preqmi antly of th frequency type and e mount o amplitude modulation is much less than that which would be present if either the copper or the powdered iron were taken out of the magnetic field of the coil. Hence, the present apparatus has the advantage that an amplitude limiting device of the type shown with similar apparatus in the copending application of Chalon W. Carnahan, Serial Number 406,431, filed August 11, 1941, now U. S. Patent No. 2,444,218, issued June 29, 1948, and assigned to the same assignee as the present application, is no longer deemed necessary for good reproduction of recordings and hence such limiting device is not shown herein in the phonograph arrangements.

Fig. 4 shows a practical construction embodying the variable inductance. A circular inductance coil l l is snugly held in a recess in vibratile element 40, which has a hole 40A (see Fig. 5) therethrough for the passage of threaded bolt 4|, which bolt rigidly joins spacers 42 and 43 and the ends of parallelly extending support members 44 and 45 with the vibratile element 49. One of the parallelly extending members 44 has a circular opening through its free end adapted to embrace and hold a solid copper cylinder 46. The other parallelly extending member 45 has a similar cylindrical opening through its free end adapted to embrace and hold a solid cylinder of powedered iron, such as is commonly formed of iron particles cemented with a suitable binder. The copper cylinder 46, circular coil l l and. cylindrical iron core 41 are mounted coaxially. The vibratile element 40, parallelly extending members 44 and 45, and spacers 42 and 43 may be of polystyrene or other similar plastic material. Vibratile element 40 is made sufiiciently long and of such small cross section so as to follow undulations in a laterally cut phonograph record, when the stylus 43 is in a groove of the laterally cut phonograph record. In other words, the arm 49 is made inherently flexible and resilient enough to allow movement of inductance coil ll relative to the cyliners 46 and 41.

As seen from Figs. 4 and 5, the vibratile element 40 has a portion 49B of reduced cross section so that the vibratile element has a large compliance-for horizontal movement of the element 40 in Fig. 4 and a small compliance for movement of the stylus 48 in the plane of the paper in Fig. 5. Thus, the arm 40, due to its shape, responds more readily to laterally cut undulations in a phonograph record than to hill and dale cut undulations in the same phonograph record.

Fig. 6 shows a tone arm in which the vibratile inductance unit shown in Figs. 4 and 5 may be mounted. The tone arm proper of Fig. 6, in conventional manner, is mounted for pivotal movement about a pivot (not shown), so that it may freely move across a record toward its center as the stylus 48 moves in a convolution or spiral groove of a record R. The tone arm shown in Fig. 6 comprises rigid supporting member 5|, which rotates around the vertical pivot (not shown) in conventional manner, and a filter element 52 connecting the supporting member 5! to a head 53. Head 53 encloses the variable inductance unit shown in Fig. 4. The unit shown in Fig. 4 may be fastened to the head 53 by means of screws 54, which are threaded in tapped holes 5'5 (Fig. 4) in the spacers 42 and 43.

In this embodiment of my invention the filter element 52 comprises a thin strip of metal lying in a vertical plane and havin its opposite ends ri idly fixed o member 5| and head 53. resp c- 7 tively. The compliance of filter element 52 bears definite relationship to the compliance of vibratile element 50, as is hereinafter explained.

The vibratile element 50 is stiff enough and the filter element 52 is weak enough, so that, taken with the mass of head 53, when the vibratile element 43 oscillates horizontally in Fig. 4 at a frequency less than a predetermined low frequency, for example, 100 cycles per second, the head 53 moves with respect to member 5I at the corresponding low frequency without relative movement between vibratile element 45 and parallelly extending members 44 and (see Fig. 4) carried by head 53. When the vibratile element moves laterally in Fig. l at a rate greater than the predetermined low frequency, the filter element 52 has not sufficient compliance and mass of head 53 is too great at those frequencies to allow head 53 to move with respect to member 5! and, as a consequence, the vibratile element 50 moves relative to the parallel extending members A l and carried by head 53 at frequencies above the predetermined low frequency.

Thus, by proper design of vibratile element 58, head 53, and filter element 52, frequencies corresponding to wow frequencies encountered in playin phonograph records are substantially eliminated without loss of fidelity.

With the tone arm construction shown in Figs. 4-5, hill and dale imperfections in a record cause little efiect on the inductance of coil i I, since the vibratile element is much more flexible for lateral movement of the element 45 in Fig. 4 than for movement in the plane of the paper in Fig. 5, and since substantially equal and opposite inductance changes are produced in coil II by copper cylinder and powdered iron cylinder 5?, even if the vibratile element 40 does move in a direction corresponding to hill and dale movement.

Since the frequency determining circuit including coil Ii is of the high capacity type, substantial changes of stray, or body, capacitances have little effect on the frequency of the wave transmitted. Consequently, if desired, some or all of the elements shown in Fig. 1 may be mounted in the tone arm head 53. The size of present day vacuum tubes or discharge devices permits the tube or device I5 of Fig. 1 to be mounted in the head of a tone arm, along with a high capacity tuned circuit, similar to the one shown in Fig. 1.

Fig. '7 shows a modified filter element of the type shown in Fig. 6. The filter elements 52A and 5213, each approximately of one-half the width of filter element 52 in Fig. 6, mechanically join the head 53 to the tone arm extension 5|, so as to produce exactly the same mechanical effect as element 52. In addition, the mechanical filter elements 52A and 52B are held in spaced relationship to one another so that they form a pair of electrical current conductors for the passage of current through the inductance coil II in the tone arm head 53.

In the arrangement shown in Figs. 8 and 9, which are sectional views of the same assembly taken at right angles to each other and on different scales, the vibratile element I50 is adapted to follow undulations in a laterally cut phonograph record. The vibratile element I55, having inductance coil i I snugly held in a circular openin therethrough, is suspended within the tone arm head, or housing, I5I by flexible connecting means I52 and I53. Each of the flexible means I52 and I53 may be a resilient wire or thin strip of metal or other resilient material, the induced stresses remaining in the range of elasticity.

In the preferred form the flexible connecting means I52 and I53 allows the vibratile element I55 in Fig. 8 to move with greater freedom in the horizontal direction than in the vertical direction, whereby the composite pickup unit moves more in response to laterally cut undulations in a phonograph record, than to imperfections in a phonograph record having indentations or mounds thereon corresponding to hill and dale variations. However, due to the arrangement of the discs and coil, the effective inductance of coil I I is unaltered by hill and dale movement. The flexible connecting means I52 and I53 may have such compliance as to allow the housing IEI, within which the element I50 is suspended, to be connected to the free end of filter element 52 in Fig. 6 in place of head 53.

When so mounted, the compliance of springs I52 and IE3, which support vibratile element I55, bears a definite relationship to the complianceof filter elements 52. That is, the mass of element I50 and the compliance of springs I52 and I53 are such that record undulations of all desired frequencies cause motion of element 558 with respect to housing I5I, but the mass of housing I5I and the compliance of spring 52 are such that, at frequencies lower than those desired and corresponding to wow in phonograph records, the housing I5I moves as a whole together with element I50, so that such low frequency undulatlons produce no relative movement between vibratile element I55 and housing I5I.

The tone arm head or housing I5I, shown in Figs. 8 and 9, has a U-shaped cross section and upon its inner walls semicircular copper and powdered iron discs are fastened by suitable means, for example, by means of screws I55. The semicircular copper and powdered iron discs are disposed so that semicircular discs of unlike material are on opposite sides of coil II at the top edge and also at the bottom edge, and so that semicircular discs of unlike material are adjacent the top and bottom coil edges on each side, as shown in Fig. 8. As the coil II rotates about its center of gravity, one coil edge approaches copper and recedes from iron while the opposite coil edge approaches iron and recedes from copper.

The arrangement shown in Figs. 10 and 11 is adapted to reproduce sound recordings from records having hill and dale impressions thereon. In this arrangement, the springs I59 and I60 have such form that the vibratile element I50 has greater flexibility for vertical movement than for horizontal movement, so that imperfections corresponding to lateral cuts on a phonograph record are reproduced with relatively small amplitude.

It is important to note that semicircular discs of like material are disposed on opposite sides of the top edge and also on the bottom edge of coil II but with semicircular discs of unlike material disposed adjacent the top and bottom coil edges on one side of element I50. As the vibratile element I50 moves in a vertical direction under the influence of hill and dale impressions in a phonograph record, up and down movement of coil II in element I50 places it alternately nearer copper and nearer iron, so that its inductance alternately decreases and increases, respectively. Furthermore, due to the arrangement of coil I I and associated semicircular discs of copper and powdered iron, th inductance of coil II remains substantially unaffected by horizontal movement of coil 9 II, since changes in inductance produced by any one of the semicircular discs is compensated by a corresponding opposite change produced by another semicircular disc.

The parts shown in Figs. 10 and 11, with the exception of flexible connecting means I59 and I60, are similar to corresponding parts shown in Figs. 8 and 9. The flexible connecting means I59 and I60 may be thin strips of metal having a greater resistance to bending along one of its axes than along its other axis. That is, the thin strip lies in a horizontal plane. When the inductance unit II is made responsive to hill and dale recordings by mounting it as shown in Figs. 10 and 11, the filter element 52 (Fig. 6) connected between the tone arm extension and head 53 has its transverse axes rotated through 90 and has such compliance with respect to the compliance of connecting means I52 and I53 that frequencies corresponding to wow frequencies are not reproduced.

In the various modifications of my invention, I have shown means for causing a predetermined variation of inductance of coil II so as to cause predetermined frequency changes in a tuned circuit as a function of linear inductance coil movement. In order to produce such changes in frequency of the tuned circuit, the corresponding variation of inductance is not necessarily linear, since the frequency of a tuned circuit varies inversely as the square root of inductance. The variation of inductance in the arrangements herein disclosed is due to the algebraic sum of two distinct variations, i. e., curve C in Fig. 3 is the algebraic sum of curves A and B, and either curve A or curve B or both curves A and B may be changed by suitably dimensioning or spacing the powdered iron and copper members which are associated with coil II. Thus, with the means herein disclosed, the effective inductance of coil I I as a function of its movement may have other desired variations than the particular variation which produces linear changes in the frequency of a tuned circuit. For instance, the efiective inductance of coil I I may be made to vary linearly with its movement.

While I have shown and described the particular embodiments of my invention, it will be obvious to those skilled in th art that changes and modifications may be made without departing from my invention in its broader aspects, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

I claim:

1. In a phonograph pickup, a supporting arm, a pickup head, a mechanical filter element connected between said arm and head, a vibratile member supported in said head, and electrical circuit connections including said filter element electrically connected with said vibratile member.

2. In a phonograph pickup, a supporting arm, a pickup head, a mechanical filter element between said arm and said head, said head including a movable coil, a stylus pivoted about an axis spaced from its record engaging tip and having mounted thereon said coil centered substantially around said axis, whereby the coil is pivoted about an axis corresponding substantially to an axis passing through a center of gravity when and as the stylus travels in the undulations of a record groove.

HENRY P. KALMUS.

REFERENCES CITED The following referemces are of record in the file of this patent:

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